Method and apparatus for making film resistors



y 5, 1959 E. R. OLSON ET AL 2,885,310

METHOD AND APPARATUS FOR MAKING FILM RESISTORS 2 Sheets-Sheet 1 FiledSept. 15, 1954 wq I F6 I g V i 4 I /Hol 1% s I I I :5 J00 I I I v I Q? Iw i I i I i L INVENTORS I I l a a rlf 0;;

l 5 -fifiem .zzce o 50 200 250 30 fi/SW gq TEMPERATURE C y 5, 1959 E. R.OLSON ETAL 2,885,310

METHOD AND APPARATUS FOR MAKING FILM liESISTORS Filed Sept. 13, 1954 2Sheets-Sheet 2 United States Patent METHOD AND APPARATUS FOR MAKING FILMRESIST ORS Earl R. Olson and Robert F. Vance, Columbus, Ohio,

assignors, by mesne assignments, to Ohmite Manufacturing Company,Skokie, 111., a corporation Application September 13, 1954, Serial No.455,417

11 Claims. (Cl. 117-227) This invention relates to film resistors, andto methods and apparatus for producing such resistors.

There are various types of resistors designed for electrical circuits,such as wire wound resistors, composition resistors and film resistors.This invention is directed to the latter type wherein a core of glass,ceramic, or other desired material is coated with the resistive filmelement.

Film type resistors as available commercially at the present time, aregenerally comprised of a core material having disposed thereon a depositof carbon or a metallic oxide. These film resistors are of limitedutility and accuracy, and are frequently marked by a short life orservice span. The films generally are unstable, and their resistivevalues vary from age, and from encounters with high temperatures whenapplied to electrical circuits. Most known film type resistors, forinstance, are subject to failure at temperatures approaching 200 C. to250 C.

The present invention is directed to the making of film resistors fromthe natural occurring transition or refractory type metals fallingwithin specified groups of the periodic table, as hereinafter set forth.The film is obtained through a process of vapor decomposition of arelatively low boiling compound of the metal, resulting in a metal filmhaving a high melting point. Thus by relatively low processingtemperatures, a high boiling, hard and well anchored resistive film isobtained.

Specifically, the transition or refractory type metals above referredto, are those naturally occurring in classes IVB, VB, V113 and VIIB ofthe periodic table. It has been found that these transition metals,produced from vapor decomposition processing, as hereinafter set forth,provide resistive films which are stable with age and use, which havegood mechanical adhesion to the core, and hardness, and which can beproduced in requisite thicknesses to provide films of practicalresistive value having a low temperature coefficient (hereinaftersometimes referred to as T.C.). The temperature coefiicient (T.C.) of aresistive film denotes the resistive value change per degree oftemperature, as heat is applied to the film by the transmission ofelectric current, or other heating means.

It is an object of the present invention to provide an improved filmtype resistor having a stable resistance over protracted periods, in useand not in use, which provides a hard surface and firm bond with thecore; and which may be produced to provide a predetermined resistancevalue having a temperature coefficient within low accepted values.

A further object of the invention is to provide an improved filmresistor which minimizes the difliculties heretofore encountered withfilm type resistors of conventional structure and design.

A further object of the invention is to provide a film resistor producedfrom the natural occurring transition metals in classes IVB, VB, VIB andVIIB of the periodic table.

A further object of the invention is to provide a film i atented May 5,1959 resistor, produced as above, by a vapor decomposition process,whereby to provide a high boiling, hard metal film through the use ofrelatively lower vapor decomposition temperatures.

A still further object of the invention is to provide an improved filmresistor, produced by vapor decomposition of the transition metalsenumerated above and using the starting materials hereinafter moreparticularly set forth.

Other objects and advantages of the invention will become apparent fromthe following description, taken in connection With the accompanyingdrawings, wherein certain preferred embodiments are set forth forpurposes of illustration.

Fig. l is a view of a film type resistor comprising the invention;

Fig. 2 is a view of a film resistor having grooves formed therein, inthe film element;

Fig. 3 is a schematic side view of the apparatus employed in producingthe film resistor in accordance with the present invention;

Fig. 4 is a schematic top sectional view of a portion of the apparatusshown in Fig. 3, more particularly illustrating the reaction chamberwherein the film deposition occurs;

Fig. 5 is a schematic view of the apparatus employed for forming themetallic chlorides, preferably used in the methods comprising theinvention; and

Fig. 6 is a chart illustrating the vapor pressure curves for thepreferred vapor decomposition starting compounds, utilized in accordancewith the present invention.

State generally, the present invention contemplates the production offilm type resistors by vapor decomposition of selected compoundscontaining the naturally occurring transition metals of classes IVB, VB,VIE and VIIB of the periodic series; the metal film being deposited ontoa ceramic body in predetermined thickness to form the resistor element.The process is carried out in a furnace apparatus including heatingmeans for vaporizing the metal compound, and heating means for heatingthe ceramic body to a relatively higher temperature onto which the metalfilm is deposited by decomposition of the compound vapor. Means isprovided for scavenging the furnace with inert gas to control thereaction; and means is provided for controlling the position of theceramic core, the vapor pressure of the compound, and

the time of operation, so as to provide a film of uniform and controlledthickness. The coated bodies thus constituted are then cut and suitablyprovided with end electrodes or terminals, to complete the resistorstructure; and if desired the resistive element may be spiralled toincrease the resistive value.

In Fig. 1 there is shown a resistor produced by the present invention.The insulating core of ceramic, glass, or other suitable material, butpreferably of steatite, is shown at 10, having the transition metalresistive coating 12 deposited thereon. Terminals are formed at theopposite ends of the resistive film by coatings of silver paint asindicated at 14 and 16, painted onto the film; there being metal bandterminals 18 and 20 embracing the silver paint upon which the terminallugs 22 and 24 are formed. Preferably a coating of lacquer or enamel 26is provided over the resistive film, for mechanical protective purposes.

In Fig. 2 a resistor structure is shown, essentially similar to thestructure of Fig. 1, except that in this instance the resistive film asindicated at Hz: is provided with a spiral groove 28 separating the filminto a spiral filament or band, whereby to increase the resistancevalue. The groove 28 may be formed in any suitable way, as for exampleby cutting a spiralled groove with a lathe, into the resistive film.

Before discussing the particular materials and methods of the presentinvention, it is desirable to discuss briefly the requisites of asatisfactory resistor, and particularly of the fihn type.

A requirement is that the resistor shall exhibit a stable or constantresistive value with age and use, including use wherein the resistor maybe repeatedly reheated to relatively high temperatures. A furtherrequirement is that the film shall have a relatively hard surface,resistive to mechanical abrasion, and that it shall adhere with tenacityto the core material so that it does not flake off in use, particularlyas the resistor is repeatedly heated and cooled. A further requirementis that the material of the film shall have a suificiently high bulkresistivity so that an excessively thin film is not required to providea resistive element of the necessary resistive value. Anotherrequirement is that the resistor shall have, for most installations, arelatively low temperature coefiicient (T.C.), viz., change in resistivevalue with temperature. A still further requirement is that theresistive film may be produced by processing steps which can be achievedand controlled, and which are not excessively expensive. It has beenfound that the resistors produced by the materials of the presentinvention, and in accordance with the process steps employed, satisfythe foregoing requirements.

It has been found that the resistivity of the transition material filmvaries inversely as the film thickness, viz., the thinner the film thehigher the resistive value. There is not a straight line relation,however, and in general as the film is made thinner, the resistivityincreases at an increasing rate. If the bulk resistivity is inadequateso that the film must be made excessively thin, the resistivity becomesunpredictable, and the film becomes unstable.

It furthermore has been found, as to the transition metals of thepresent invention, that the temperature cefficient (T .C.) of the filmvaries with the film thickness, a film thickness of predetermined valueproviding a substantially zero T.C., films of less thickness providing anegative T.C. which increases as the film thickness decreases, and filmsof greater thickness providing a positive T.C. which increases as thefilm thickness increases. It is desirable that the cross-over point,viz., the film thickness providing a substantially zero T.C., shall besufiiciently wide in range so that such desired film can be practicablyproduced. It is furthermore required that the resistivity of the film atsuch cross-over point thickness shall be sufliciently high to provide apractical resistor.

It has been found that the transition metals of the present inventionprovide film resistors satisfying the foregoing requirements as to filmthickness and cross-over range. Further, in accordance with the presentinvention apparatus and methods are provided for controlling theproduction of the film, so that the film thickness can be controlled toprovide a resistor of desired resistance value, and of substantiallyZero T.C. In some installations it may be desirable that the resistorprovide a changing resistance value with temperature change, viz., apositive or negative T.C.; but in most installations it is desirable toprovide a resistor the resistive value of which remains substantiallyunchanged with changes of temperature, or which has a substantially zeroT.C. Film resistors as heretofore produced have not compared favorablywith wire wound resistors in this respect. On the other hand, wire woundresistors become excessively expensive in the high resistance valueranges. The resistor as provided by the present invention satisfies bothrequirements.

Referring more particularly to the transition elements in classes IVE,VB, VIB and VIIB of the periodic table, technetium is not a naturallyoccurring element. Hafnium and niobium (columbium) are not readilyavailable commercially, and while operative, are therefore of lesserimport. Of the remaining nine transition metals in classes IVB, VB, VIBand VIIB, tungsten (Wolfram), tantalum,

4 molybdenum, and vanadium constitute a preferred subgroup (as comparedwith the remaining elements titanium, zirconium, chromium, manganese andrhenium); and within the preferred subgroup tungsten, tantalum,molybdenum and vanadium are preferred in the order stated, because oftheir properties.

As stated, the films of the foregoing metals, in accordance with thepresent invention, are produced by the process of vapor decomposition,from the starting composition comprising the metal. The startingcompounds may comprise the poly-halides, carbonyls, or hydrides, of themetals, or organic compounds containing the metals. Of these thepoly-halides, and specifically the chlorides, are preferred because oftheir availability, volatility, and susceptibility to vapordecomposition. The vapor decomposition n.ay be effected by directpyrolysis, but in the case of the halides preferably the decompositionis facilitated by reduction with hydrogen, as will be more specificallyhereinafter discussed.

The apparatus by which the film deposition is effected is illustrated inFigs. 3 and 4. Referring to Fig. 3, there is illustrated a reactionchamber or furnace 3t and a drive mechanism for controlling themovements of the core material or substrate to be coated within thefurnace, generally indicated by the reference numeral 32.

The drive mechanism more specifically comprises a carriage 34 actuatedalong a predetermined path of longitudinal travel, and at predeterminedspeed, by an elongated drive screw 36. The carriage 34- is provided witha half nut 38 selectively engageable with the drive screw by means of acontrol handle 40, the arrangement being such that when the half nut isin engagement with the screw, upon screw rotation the carriage 34 willbe driven.

Screw 36 is journalled at its opposite ends in a pair of supportuprights 42 and 44, and is arranged to be driven through a reversible,variable speed motor 46 and a transmission 48. By means of the motor thescrew may be driven at varying rates of speed, and selectively in eitherdirection. The control circuit for the motor includes limit switches 54)and 52., for preventing over travel of the carriage 34 in eitherdirection of movement. The carriage is guided by means of guide rods, asindicated at 54 and 56.

Carriage 34 carries a variable speed motor 58 arranged to driverotatably, a core or substrate holder 60 arranged to project into thefurnace 3t).

Referring to Fig. 4, wherein the furnace or reaction chamber is moreparticularly shown, it will be seen that the furnace comprises a chamber62, within which is mounted a pair of Globars or heating elements 64 and66, the control circuit for one of which is shown at as. By means ofthese heating elements the substrate within the furnace may be heated toa desired deposition temperature.

A deposition chamber is provided within the furnace by a tube 72, intothe opposite ends of which project elongated tubes 74 and 7 6, whichlatter tubes also project through the end walls of the furnace. Tube 74is provided at its end with a removable cap 78 providing a bearing St)for the rotatable core holder 60, whereas tube 76, which projectsthrough the opposite or discharge end of the furnace, is provided with apair of large bore stopcocks 32 and 84. To provide an inert atmospherewithin the tubes, tube 74 is provided with an inlet conduit 86 for theintroduction of an inert scavenging gas such as helium, under control ofa valve 88, and tube 76 is similarly provided with a helium inletconduit 99 controlled by valve 92. The helium gas may pass through theopen ends of the tubes 74 and 75, into the deposition tube 76, asindicated by the arrows, and tube 76 is provided also with an exhaustconduit or pipe 94 adjacent the stopcock 84, controlled by a valve 96.

Associated with the furnace 30 is a pair of lower temperature vaporizingchambers 109 and 102, provided with heating elements as indicated at164, 1'36, 108

and 110, the control for one of which is shown at 112.

Chambers 100 and 102 are provided, respectively, with trays 114 and 116into which is placed the transition compound to be decomposed, thesetrays communicating with conduits 118 and 120 leading to the reactiontube 70, and being provided with inlet pipes or conduits 122 and 124,under control of valves 126 and 128, respectively, for the introductionof a reacting gas, specifically hydrogen as a reducing gas, in theembodiment set forth.

The reaction tube 72 is further provided with a pair of exhaust conduits130 and 132, under control of valves 134 and 136, respectively.

In preparing a typical film deposit, the starting material containingthe transition metal, for example tungsten hexachloride, is introducedinto the trays 114 and 116. The substrate or core rod, of steatite,Vycor, or the like, indicated by the reference numeral 140 in Fig. 4, issecured to the chuck 142, provided at the end of the rotatable holder60.

Upon operation of the drive motors 46 and 58, Fig. 3, the core rod 140will be rotated and longitudinally advanced, at a predetermined rate, tothe right as shown in Fig. 4. Upon application of heat within thevaporizing chambers 100 and 102, the tungsten hexachloride will bevaporized, and propelled by its vapor pressure, along with hydrogen,into the deposition chamber 70 from the conduits 118 and 120.

As will be understood, upon the application of a higher degree of heatto the core 140, by the action of the heating elements 64 and 66, as thetungsten hexachloride contacts the surface of the rod, a reducing actionor vapor decomposition takes place, the tungsten being deposited as ametal film upon the surface of the core rod.

It will be noted that as the core approaches the deposition zone withinthe inlet tube 74, it is preheated while being maintained within theinert atmosphere of helium within the tube. Similarly, as the coreleaves the deposition zone, it moves into the tube 76 wherein it isagain subjected to an atmosphere of inert helium. This protection of thefilm as it leaves the reaction chamber is desirable to preventcontamination of the film with partially reduced metallic chloride. Withproper balance of the volumes of hydrogen and helium, and the pressuresthereof, a protective helium atmosphere may be maintained within thetubes 74 and 76, without undue dilution of the reactive gases within thedecomposition chamber.

It will furthermore be seen that by proper control and operation of thedrive motors 46 and 58, not only is a uniform film deposited on the core140, as the core is rotated and longitudinally translated through thereaction Zone, but by controlling the speeds of movement, in relation tothe vaporizing temperatures existing within the chambers 100 and 102,the thickness of the metallic film which is produced may be accuratelycontrolled.

After the coated substrate 140 has been projected into the coolingsection 144 of the tube 76, the chuck 142 of the holder is released bymanipulation of a control handle 146, Fig. 3, whereupon the holder maybe retracted and the operation repeated. The coated core or substratecools within the tube section 144, in a helium atmosphere, and whencooled may be withdrawn through the stopcock 84.

The exhaust gases from the exhaust tubes 130 and 132 may be vented to arecovery chamber, for recovery of any unexpended metallic chloride.

It will be seen that the vapor decomposition process, as abovedescribed, comprises the vaporization of the transition elementcompound, by the application of willcient heat to effect vaporization,and the subsequent decomposition of the compound upon contact with themore highly heated substrate, whereby to provide the metallic filmthereon. This process is in contrast with the direct vaporization ofmetals, at high temperature, and under vacuum, where temperatures mustbe employed equal to or greater than the vaporizing temperatures of themetal films.

Proper control of the temperature of the substrate is necessary toproduce a properly adherent film. If the temperature of the substrate isexcessive, the films deposited thereon will be colored and notreproducible, whereas if the temperature of the substrate is too low,the deposited films will be non-adherent. Proper temperature conditionsare those resulting in a film which is silver-like in appearance, andstrongly adherent to the core material. The flow rate of hydrogen, inthe case of decomposition by hydrogen reduction, also reflects upon thethickness of the film produced, for any given core transmission speed.

Conditions are set forth in the following table, as to films which havebeen produced from chlorides of the transition metals indicated.

Conditions of deposition for transition metal films Temp. of ReactionReaction H Flow Metal Chloride, Temp., Time, Rate,

0. 0. Min. cc./rnin Titanium 1-27 500-1, 000 10-15 2, 000-3, 800Zirconium 275-365 950-1, 135 4-15 2, IOU-6,900 Vanadium Room 950-1, 000l-5 3, 900-7, 900 Tantalum -250 750-1,000 5-15 2, 9008,600 Molybdenum--140 850-900 0.5-5 9,000 Tungsten 775-800 6 to 20 6 900-8, 700 Ti-ZrCodeposi Rggm Z0131) 580-800 5-10 3, 100-5, 000

5 r Tl-Va Room 875-925 2-5 3, 900

As examples, excellent tungsten films were obtained when tungstenhexachloride at a temperature of 150 C. was introduced into the reactionchamber heated to 77 5- 800 C. for a time interval of six to twentyminutes, during which period a volume of 6900 to 8700 cc./min. ofhydrogen was caused to flow.

As further illustrative, a tantalum film was obtained by subjecting asteatite core at temperatures ranging from 750-1000 C. to a vapor oftantalum pentachloride vaporized at temperatures of 100-250 C., for fiveto fifteen minutes in a hydrogen flow of 2900-8600 cc./min.

The reaction that is obtained may be described generally by the formula:

Where M refers to the transition metal under consideration, and X is thenumber of chloride atoms necessary to satisfy the highest valence of themetal. From the apparatus and process provided, the flow system involvesan unrestricted volume, and the reaction normally will go to completion.

As previously pointed out, the resistance value of the metal filmdepends upon the bulk resistivity of the metal, and the film thickness.The resistance value of the film is expressed as resistance per square(R/sq.), which is a constant for a given metal and film thickness. Asfurther previously pointed out, the temperature coefiicient (T.C.) ofthe film varies with the film thickness; and in accordance with themethods and apparatus heretofore set forth, the film thickness can becontrolled to produce a substantially zero T.C., or a negative T.C. ofgiven value, or a positive T.C. of given value, by controlling thethickness of the deposited film. The T.C. of any given film can bedetermined by actual test, as will be understood. In films which havebeen produced, a resistance of about 3000 ohms per square characterizedtungsten films at a zero T.C.; whereas tantalum films at zero T.C. coveran approximate resistance range of 100- 500 ohms per square.

After a film thickness has been produced, to provide a desiredtemperature coefiicient, the coated core material may then be cut toproper length to provide the desired total resistor value for the unit,and if desired or necessary, the deposited film may be spiralled as indicated in Fig. 2, to increase the resistance value. The bulk resistivityof the transition metals within classes IVB, VB, VIB and VIIB of theperiodic table varies rather widely, so that a selection of resistivefilms is provided. The melting points are quite high, contributing tothe usefulness of the resistive films produced. The films may becodeposited, as indicated in the chart heretofore set forth.

In certain instances the stability of the films has been increased bythe preheating thereof, and it is within the contemplation of thepresent invention that the films, produced as above set forth, may beheated, and reheated, and maintained at elevated temperatures for aprotracted period, prior to use, to increase the stability of theresistive films.

As previously stated, within the group classification set forth,tungsten films have exhibited exceptional qualities in respect to aging,stability under load, and stability of temperature coeflicient; alongwith films of tantalum, molybdenum and vanadium in the preference orderstated.

The following table is representative of tungsten films of variousthicknesses produced from tungsten hexachloride:

Molybdenum pentachloride was similarly prepared by direct synthesis fromthe elements. In a representative run a charge of 95 grams of molybdenumpowder, 99.9% pure, was used in reaction with the chlorine. The reactioncommenced at 300 C. and attained a satisfactory rate of 450 C.Approximately 164 grams of blue-black molybdenum pentachloride wereobtained, representing a yield of 61% of theoretical.

Titanium tetrachloride may be obtained commercially. For use in thepresent invention commercial titanium tetrachloride was purified by arefluxing process for six hours in contact with by weight of coppergauze. After standing overnight, the tetrachloride was distilled througha Vigreux column, the fraction boiling at 131- 132 C. being retained.

For use in the present process, anhydrous chromic chloride (CrCl wasprepared by heating hydrated chromic chloride in the presence of carbontetrachloride. A quantity of the CrC1 -6H O was placed in a flask andthen into a hot furnace, and when the temperature reached 150 C., carbontetrachloride was introduced at a rate of two or three drops per second.At 300 C. phosgene evolution commenced. About 400 milliliters of CCL,were required to dehydrate 148 grams of Temp. of Vapor Reaction ReactionHydrogen Helium Speed of Run Number W01 Pressure Temp., Time, Flow FlowSubstrate Remarks C. of W01 C. min. Rate, Rate, Rotation,

mm. of ccJmin. cc./min. r.p.m.

150 25 800 6, 900 100 1 Silver appearance. 150 800 20 6, 900 100 1 Do.150 25 800 15 6, 900 100 1 D0. 150 25 800 9. 5 8,700 110 6 Do. 150 25800 9. 5 8, 700 110 6 Do. 150 25 800 9. 5 8, 700 110 6 Do. 150 25 775 9.5 8, 700 110 6 D0. 150 25 775 8. 0 8, 700 110 6 D0. 150 25 775 6. 0 8,700 110 6 Do.

As examples of starting materials, the preparation of the polychlorideswill be particularly discussed.

In the preparation of zirconium tetrachloride, for instance, apparatussuch as that schematically outlined in Fig. 5 may be and has beenemployed; as well as for the preparation of tantalum pentachloride,molybdenum pentachloride and tungsten hexachloride.

In preparing zirconium tetrachloride, zirconium of good purity may beplaced into a reaction tube 150, Fig. 5, preferably fashioned from Vycortubing, associated with a resistance wound tube furnace 152. Athermocouple 154 may be employed for controlling the temperature of thereaction. Argon gas is introduced into the reaction system through aconduit 156 as the furnace is brought to reaction temperature. When suchtemperature is reached chlorine gas is introduced through conduit 158,and passed at a moderate rate over the zirconium, the resultantzirconium tetrachloride being collected into a receiving flask 160,maintained at cool temperatures. While the reaction begins at 300 C., itis preferably conducted at a temperature of approximately 425 C. Therate of chloride formation at such temperature, in a representative runmade, yielded 149 grams of zirconium tetrachloride in a period of tenhours, which was approximately a yield of 80% of theoretical.

Similar procedure is followed in the preparation of tungstenhexachloride by direct synthesis of its elements at 770 C. Tungstenchips greater than 99% purity are used, and a yield of 90% oftheoretical is obtained.

Tantalum pentachloride is prepared by the direct combination of tantalumchips with dry chlorine. In a representative run, in two hours a chargeof grams of tantalum chips yielded 20 grams of tantalum pentachloriderun at a temperature of 250 C. This is 43% of the theoretical yield.

CrCl 6H O. The temperature was raised slowly throughout the reaction.Final temperature was 650 C. A yield of 77 grams of chromic chloridecrystals was obtained, which is 88% of the theoretical yield.

Anhydrous manganous chloride (MnCl was prepared by a Vacuum dehydrationof the tetrahydrate (MnCl 4H O) A quantity of the dehydrate of 200 gramswas ground and placed in a flask heated by an electric mantle. The flaskwas evacuated, the water being collected in an ice trap as it evolved.The powder was heated for two hours at 240 C., and then thirteen hoursat 350 C. The yield of anhydrous MnCl was 124 grams, which was 97%theoretical.

Vanadium tetrachloride (VC1 was prepared by direct combination ofvanadium and chlorine. Chips of vana dium of 99.8% purity were placed ina flask and heated by an electric mantle. Dry chlorine was introduceddirectly over the vanadium via a long tube through a distillation head.The head was connected to a receiving flask by a long glass tube, andthe receiving flask was cooled by a slurry of ice and water, and wasopen to the atmosphere through a drying tube containing calciumchloride. The reaction occurred at 350 C., the red VCl distilling intothe receiving flask as it was formed, the vertical section of the stillhead being heated to prevent reflux of the VC14. A charge of 53 grams ofvanadium yielded 200 grams of VCL; in six hours, which was 99% oftheoretical.

In Fig. 6 the vapor pressure curves are shown for tungsten hexachloride,tantalum pentachloride, molybdenum pentachloride, and vanadiumtetrachloride, which are the preferred starting materials for thepreferred transition elements as hereinbefore set forth and as hereinused.

Changes may be made in the form, construction and arrangement of theapparatus from that disclosed herein, and in the method steps outlined,without departing from the spirit of the invention. The invention isaccordingly not to be limited to the specific embodiments set forth, butonly as indicated in the following claims.

The invention is hereby claimed as follows:

1. The method of making a film resistor which comprises vaporizing achloride selected from the group consisting of the chlorides oftantalum, molybdenum, tungsten and vanadium and mixing said vaporizedchloride with hydrogen while maintaining said vaporized chloride at atemperature below that temperature at which said chloride will bedecomposed to form a free metal; heating a dielectric core to atemperature at which said chloride will be reduced by said hydrogen; andcontacting said vaporized chloride-hydrogen mixture with said heatedcore whereby said chloride is reduced at the surface of said core andsaid core is coated to form a resistive film thereon.

2. A method according to claim 1 in which said chloride is vaporized inthe presence of hydrogen.

3. A method according to claim 1 in which said core is heated in aninert atmosphere prior to said contacting step.

4. A method according to claim 1 in which said core is cooled in aninert atmosphere subsequent to said contacting step.

5. A method according to claim 1 in which said core is subjected to aheat treatment for stabilizing purposes subsequently to the coating ofsaid core.

6. A method according to claim 1 in which said chloride is vanadiumchloride; in which said vaporized vanadium chloride is maintained atabout room temperature; and in which said core is heated to about 950 C.to 1000 C.

7. A method according to claim 1 in which said chloride is tantalumchloride; in which said vaporized tantalum chloride is maintained atabout 100 C. to 250 C.; and in which said core is heated to about 750 C.to 1000 C.

8. A method according to claim 1 in which said chloride is molybdenumchloride; in which said vaporized 10 molybdenum chloride is maintainedat about C. to C.; and in which said core is heated to about 850 C. to900 C.

9. A method according to claim 1 in which said chloride is tungstenchloride; in which said vaporized tungsten chloride is maintained atabout C.; and in which said core is heated to about 775 C. to 800 C.

10. Apparatus for making film resistors which comprises means defining aclosed treating chamber penneable to radiant heat and adapted to receivea core for a film resistor, a source of radiant heat outside saidtreating chamber for heating a core received in said treating chamber,means defining a closed vaporizing chamber permeable to radiant heat andadapted to receive a coating material to be vaporized, a source ofradiant heat outside said vaporizing chamber for heating coatingmaterial received therein, means for conducting said vaporized coatingcompound from said vaporizing chamber to said treating chamber, andmeans communicating with one of said chambers for mixing hydrogen withsaid vaporized coating compound.

11. Apparatus according to claim 10 additionally comprising meansdefining preheating and cooling chambers adapted to receive said core,respectively, before and after said core is coated in said treatingchamber, and means for supplying an inert gas to said preheating andcooling chambers.

References Cited in the file of this patent UNITED STATES PATENTS1,399,722 Heany Dec. 6, 1921 1,497,417 Weber June 10, 1924 1,923,845Rentschler Aug. 22, 1933 1,965,059 Seibt July 3, 1934 2,047,351Alexander July 14, 1936 2,183,302 Brauer et a1. Dec. 12, 1939 2,357,473Jira Sept. 5, 1944 2,382,432 McManus et a1. Aug. 14, 1945 2,418,804 HoodApr. 8, 1947 2,656,284 Toulmin Oct. 20, 1953 2,667,432 Nolte Jan. 26,1954 2,671,735 Grisdale et a1. Mar. 9, 1954 2,698,812 Schladitz Jan. 4,1955

1. THE METHOD OF MAKING A FILM RESISTOR WHICH COMPRISES VAPORIZING ACHLORIDE SELECTED FROM THE GROUP CONSISTING OF THE CHLORIDES OFTANTALUM, MOLYBDENUM, TUNGSTEN AND VANADIUM AND MIXING SAID VAPORIZEDCHLORIDE WITH HYDROGEN WHILE MAINTAINING SAID VAPORIZED CHLORIDE AT ATEMPERATURE BELOW THAT TEMPERATURE AT AWHICH SAID CHLORIDE WILL BEDECOMPOSED TO FORM A FREE METAL; HEATING A DIELECTIC CORE TO ATEMPERATURE AT WHICH SAID CHLORIDE WILL BE REDUCE BY SAID HYDROGEN; ANDCONTACTING SAID VAPORIZED CHLORIDE-HYDROGEN MIXTURE WITH SAID HEATEDCORE WHEREBY SAID CHLORIDE IS REDUCED AT THE SURFACE OF SAID CORE ANDSAID IS COATED TO FORM A RESISTIVE FILM THEREON.